Optical filters are often used to select at least one optical frequency band, called a passband, out of an optical frequency spectrum of an optical signal. A central frequency of the passband of a tunable optical filter is adjustable, depending upon a control parameter common to a particular filter type. For example, for a bulk optic tunable filter, the control parameter can be a filter tilt or a clocking (rotation) angle with respect to an incoming optical beam. For an optical waveguide based tunable filter such as tunable Mach-Zehnder (MZ) interferometer, the control parameter can be an electrical signal applied to a localized heater that changes the optical path length of one of its arms, which effectively tunes the MZ interferometer.
Tuning range, spectral selectivity, and a level of cross-talk suppression are very important parameters of tunable optical filters. A wide tuning range allows a wide range of optical frequencies to be accessed and selected by a tunable filter. The spectral selectivity relates to an ability of the filter to select a narrow frequency band of a broadband optical signal. Herein, the term “narrow” means small as compared to a value of the central frequency of the optical signal being filtered, for example 1% of the central frequency or less. Finally, the crosstalk suppression is an ability of the filter to suppress optical signals at any other frequency than the frequency of the signal being selected.
In an optical communications network, optical signals having a plurality of optical channels with different optical frequencies or wavelengths called optical frequency channels or wavelength channels, are transmitted from one location to another, typically through a length of optical fiber. Optical frequency channels can be combined for transmission through a single optical fiber, whereby the transmission capacity of the optical fiber increases many times. Since the optical frequency channels can be amplified simultaneously in a single optical amplifier, the transmission distances are increased, while the associated transmission costs are considerably reduced.
Tunable optical filters are used in optical communications networks for selecting one or more optical frequency channel out of a plurality of channels comprising the optical communications signal. Tunable optical filters are also used for system performance monitoring purposes, e.g. for performing a spectral measurement of the entire optical communications signal, including measuring optical noise levels between the neighboring frequency channels. The tunability of the filter allows any optical frequency component within the tuning range of the filter to be selected for subsequent detection and/or signal level measurement. Ideally, a tunable filter has excellent crosstalk suppression, since poor crosstalk suppression leads to undesired “leaking” of the optical channels being suppressed, thus impairing the signal level measurements and/or detection and decoding of the selected signal.
U.S. Pat. No. 5,596,661 entitled “Monolithic Optical Waveguide Filters based on Fourier Expansion”, issued to Henry et al., and incorporated herein by reference, teaches a planar lightwave circuit (PLC) optical filter having a chain of optical couplers linked by different delays with a transfer function equal to the sum of the contribution from each optical path, with each contribution forming a term in a Fourier series whose sum forms the optical output. Detrimentally, the optical filter of Henry et al. is not tunable.
U.S. Pat. No. 6,208,780 entitled “System and Method for Optical Monitoring”, issued to Li et al., and incorporated herein by reference, teaches a tunable optical filter on a PLC chip using cascaded unbalanced Mach-Zehnder (MZ) interferometers. In the tunable filter of Li et al., successive MZ stages have twice the free spectral range (FSR) as the previous MZ stages, thereby providing a narrowband optical filter having a wide tuning range. Unfortunately, the tunable optical filter requires many MZ stages, including stages that have to be repeated, to achieve a satisfactory crosstalk suppression.
U.S. Pat. No. 8,340,523 entitled “Tunable optical filter”, issued to Shen et al., hereby incorporated by reference herein, teaches a tunable optical filter on a PLC chip having sequentially connected thermally tunable MZ interferometers having different FSRs. To achieve a high level of crosstalk suppression, each of the MZ interferometers is tuned so as to have one passband of each MZ interferometer centered on the central frequency of the single frequency channel being selected, and at least one of the stopbands of the MZ interferometers centered on a central frequency of each remaining optical frequency channel of the optical signal. In contrast to the tunable filter taught by Li et al., the tunable optical filter taught by Shen et al. includes MZ interferometers having FSRs that are an integral number the frequency grid. The resulting optical filter has a crosstalk that is improved by at least two orders of magnitude relative to the crosstalk performance of the filter disclosed in U.S. Pat. No. 6,208,780.
Notably, the tunable optical filter taught in U.S. Pat. No. 8,340,523 has a low insertion loss and Gaussian passband shape. In general, a flat-top passband is preferred to a Gaussian passband, since it provides a wider passband and is less likely to alter the optical signal. In order to improve the spectral shape of the passpand, Shen et al. disclose an embodiment having a interleaver stage including first and second MZ interferometers. These MZ interferometers are tuned to maximum transmission at the filter wavelength, and have a FSR that is an integral number of the frequency grid spacing. While this interleaver stage has been shown to provide a wider passband and a steeper roll-over, the bandpass is still substantially Gaussian-like in shape.